Spine stiffening device

ABSTRACT

In various embodiments, a spinal stabilization system has a first and a second vertebral anchor each adapted to be coupled to one of a first and a second vertebrae. The system also includes a flexible connecting element coupled to each of the first and second vertebral anchors and extending between the vertebral anchors. The connecting element has a central member and a brace positioned about the central member and the central member is stiffer than the brace to provide the needed combination of support and stiffness to the construct while offering flexibility. These and other various embodiments of this invention offer an improvement over known systems by providing stabilization of the vertebral column while allowing for flexibility and axial dampening.

TECHNICAL FIELD

This invention relates generally to therapeutic and corrective devicesand methods for the spine and more specifically relates to devices andmethods for stiffening the spine.

BACKGROUND

The spine includes a series of joints routinely called motion segmentunits, which is the smallest component of the spine that exhibitskinematic behavior characteristic of the entire spine. The motionsegment unit is capable of flexion, extension, lateral bending andtranslation. The components of each motion segment unit include twoadjacent vertebrae and their apophyseal joints, the intervertebral disc,and the connecting ligamentous tissue. Each component of the motionsegment unit contributes to the mechanical stability of the joint.

Components of a motion segment that move out of position or becomedamaged can lead to serious pain and may lead to further injury to othercomponents of the spine. Depending upon the severity of the structuralchanges that occur, treatment may include fusion, discectomy, orlaminectomy.

Underlying causes of structural changes in the motion segment unitleading to instability include trauma, degeneration, aging, disease,surgery, and the like. The purpose of fusion or rigid stabilization isthe immobilization of a motion segment unit. Thus, fusion or rigidstabilization of one or more motion segment units may be an importantelement of a surgical procedure in certain cases (i.e., injuries,deformities, tumors, etc.). In other cases it is a complementary element(i.e., fusion performed due to degeneration).

One current spinal surgical technique typically involves fusing one ormore unstable motion segment units and possibly, the removal ofligaments, bone, disc, or combinations thereof included in the unstablemotion segment unit or units prior to fusing. However, fusion ofteninvolves several disadvantages. For example, the fusing process resultsin a permanent or rigid internal fixation of all or part of theintervertebral joints and usually involves metallic rods, plates, andthe like for stabilization. Such systems are intended to rigidlyimmobilize the motion segment unit to promote fusion within that motionsegment unit resulting in a loss of mobility.

Fusion also causes the mobility of the motion segment to be transferredto other motion segments of the spine. The added stresses transferred tomotion segments neighboring or nearby the fused segment can cause oraccelerate degeneration of those segments. Another disadvantage offusion is that it is an irreversible procedure.

Spinal fixation systems using orthopedic rods as described above providea relatively rigid construct. Another type of spinal treatment techniqueoffers dynamic stabilization for the spine. One example of a dynamicstabilization system is provided by the assignee of this invention underthe trademark DYNESYS® as generally described in European PatentApplication No. 0669109A1 which is hereby incorporated by reference inits entirety. Such a dynamic stabilization system utilizes pediclescrews installed into appropriate locations in adjacent vertebrae.Flexible materials in conjunction with the pedicle screws are usedrather than rigid orthopedic rods or bone grafts alone as an adjunctfusion. A tubular spacer is positioned between the pedicle screws onadjacent vertebrae with a tensioned cord passing through the centrallumen of the spacer. Dynamic stabilization systems of this type bringthe lumbar vertebrae into a desired anatomical position whilestabilizing the effective segments and without irreversible fusion.

Fusion procedures can be improved by modifying the load sharingcharacteristics of the treated spine and one technique is to allow moreof a physiologic loading between pedicular fixation and anterior columnsupport. Additionally, a device and associated method that precludes orat least delays the need for fusion for all but the most advanceddegeneration of a motion segment, particularly if such a device wouldallow close to normal motion and pain relief is desirable. Moreover,utilization of minimally invasive surgical procedures and techniques toinstall such devices is also a highly desirable objective to enhance andpromote patient healing and recovery.

SUMMARY OF THE INVENTION

This invention addresses these and other shortcomings in the prior art.In various embodiments, the invention includes a spinal stabilizationsystem having a first and a second vertebral anchor each adapted to becoupled to one of a first and a second vertebrae. The system alsoincludes a flexible connecting element coupled to each of the first andsecond vertebral anchors and extending between the vertebral anchors.The connecting element has a central member and a brace positioned aboutthe central member and the central member is stiffer than the brace toprovide the needed combination of support and stiffness to the constructwhile offering flexibility.

In other embodiments of this invention, the spinal stabilizationconstruct includes first and a second top loading polyaxial pediclescrew each adapted to be coupled to one of a first and a secondvertebrae. The construct also includes a flexible connecting elementcoupled to each of the first and second pedicle screws and extendingbetween the pedicle screws. The connecting element has a central member.A first and a second securing member are each threadably coupled to oneof the pedicle screws to secure the connecting element thereto. A braceis positioned concentrically about the central member and the centralmember is stiffer than the brace. A first and a second shank extend inopposite directions from the central member and each shank is coupled toone of the pedicle screws. A first and a second flange are on theconnecting element and the flanges are spaced on opposite sides of thebrace.

The spinal stabilization system may also include a third and a fourthflange on the connecting element, each of which are spaced on oppositesides of the central member, embedded in the brace and spaced from thefirst and second flanges.

Other embodiments of a spinal stabilization system according to thisinvention include a first and a second vertebral anchor each adapted tobe coupled to one of a first and a second vertebrae and a flexibleconnecting element coupled to each of the first and second vertebralanchors and extending between the vertebral anchors. The connectingelement has a central member and a brace positioned about the centralmember. A first and a second flange are each on the connecting elementand spaced on opposite sides of the brace. The system may also include afirst and a second shank extending in opposite directions from the firstand second flanges, respectively. Each shank is coupled to one of thevertebral anchors. A third and a fourth flange may be included on theconnecting element and spaced on opposite sides of the central member,embedded in the brace and spaced from the first and second flanges. Thefirst and second flanges may be integrally formed with the first andsecond shanks, respectively.

In another embodiment of this invention, the spinal stabilizationincludes a first and a second polyaxial pedicle screw each adapted to becoupled to one of a first and a second vertebrae. A flexible connectingelement is coupled to each of the first and second polyaxial pediclescrews and extends between the pedicle screws. A first and a secondsecuring member are each threadably coupled to one of the pedicle screwsto secure the connecting element thereto. The flexible connectingelement may be integrally formed from a polymeric, elastomeric ornon-metallic material.

These and other various embodiments of this invention offer animprovement over known systems by providing stabilization of thevertebral column while allowing for flexibility and axial dampening.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is perspective view of a spinal stabilization construct accordingto a first embodiment of this invention;

FIG. 2 is a side elevational view of the system of FIG. 1 beinginstalled on a patient's vertebral column;

FIG. 3 is a cross-sectional view of a flexible connecting element of theembodiment of FIG. 1;

FIG. 4 is a cross-sectional view of another embodiment of a flexibleconnecting element according to this invention;

FIG. 5 is a cross-sectional view of a still further embodiment of aflexible connecting element according to this invention;

FIG. 6 is a perspective view of a multi-level spinal stabilizationsystem according to another embodiment of this invention;

FIG. 7 is a perspective view of a still further embodiment of a flexibleconnecting element for use in a spinal stabilization system according tothis invention;

FIG. 8 is cross-sectional view of the flexible connecting element ofFIG. 7;

FIG. 9 is a perspective view of a still further embodiment of a flexibleconnecting element for use in a spinal stabilization system according tothis invention;

FIG. 10A is a disassembled, perspective view of the flexible connectingelement of FIG. 9;

FIG. 10B is a partially disassembled view of the spinal stabilizationsystem utilizing the flexible connecting element of FIGS. 9 and 10A;

FIG. 11 is a cross-sectional view of the assembled spinal stabilizationsystem of FIG. 10B; and

FIG. 12 is a view similar to FIG. 11 of a multi-level spinalstabilization system.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, one embodiment of a spine stiffening stabilizationsystem 10 is shown and includes a connecting element 12 coupled to andextending between a pair of vertebral anchors 14. Each vertebral anchor14 is adapted to couple to either of two vertebras 16 in a patient'sspine 18. With the connecting element 12 secured to the vertebralanchors 14, the connecting element 12 maintains a restored disc spaceheight and/or alignment between the adjacent vertebrae 16. In one form,the connecting element 12 preserves motion between the adjacentvertebrae 16.

In one embodiment as shown in the various drawings, such as FIGS. 1, 2,10B, and 11, the vertebral anchor 14 is a polyaxial, top loading pediclescrew including a threaded shaft 20 adapted to be inserted into apedicle area 22 of the vertebrae 16. The vertebral anchor includes ahead 24 on the threaded shaft 20 and a saddle 26 is coupled to the head24 and is adapted for polyaxial movement of the saddle 26 relative tothe shaft 20. The saddle 26 having an exemplary U-shape provides a toploading capability to the vertebral anchor 14 so that the connectingelement 12 may be inserted downwardly into the saddle 26 between a pairof polyaxial head receiving arms 28. The saddle 26 can be constructedwith any desired shape. A securing member in the form of a threaded setscrew 30 mates with the saddle 26 to secure the connecting element 12 tothe vertebral anchor 14. Each receiving arm 28 of the U-shaped saddle 26has threads 32 which mate with the threads 34 on the set screw securingmember 30. While a top loading polyaxial pedicle screw is shown anddescribed herein as one embodiment of the vertebral anchor 14, othertypes of vertebral anchors can readily be used within the scope of thisinvention including standard fixed pedicle screws, hooks or otherdevices intended to couple a connecting element to the vertebrae.

Referring to FIGS. 1-3, one embodiment of the connecting element 12according to this invention includes a central portion or member 36 anda pair of connecting portions or shanks 38 that include rod portions 39that can be of any suitable shape to fit within the saddle 26 extendingin opposite directions on the connecting element 12. Each shank 38 iscoupled to one of the vertebral anchors 14. The terminal ends of theshanks 38 of the various embodiments of this invention can be trimmed tolength after sizing and installation. Alternatively, the connectingmembers 38 can be pre-sized prior to implementation. The central member36 is concentrically surrounded by a flexible portion or brace 40. Thebrace 40 is positioned between spaced flanges 42 and the central member36 is secured within the brace 40 and between the flanges 42 by crimpingtabs 44.

The connecting element 12 of this invention includes a flexibleintermediate portion 46. In one embodiment, the brace 40 and centralmember 36 are intended for use in the intermediate portion 46.Alternatively, a solid flexible core can be used as the flexibleintermediate portion 46. In one embodiment, the brace 40 is anelastomeric member that allows for flexing of the connecting element 12.The brace 40 stabilizes the spine 18 while allowing for flexibility andaxial dampening. In one embodiment, the central member 36 is tensionedand integral to the intermediate portion 46 of the connecting element12. The central member 36 allows for bending of the shanks 38 and theassociated forces on the brace 40. Unlike standard spine rods whichprovide little or no flexibility and axial dampening, the connectingelement 12 of this invention allows for flexing of the spinalstabilization system 10 in response to movement of the vertebrae 16. Theconfiguration and interaction of the various component parts of theconnecting element 12 of FIGS. 1-3 allows for bending of the connectingelement 12 and the shanks 38 connected to the vertebral anchors 14 sothat the central member 36 is placed under tension and one side of thebrace 40 is compressed depending upon the orientation of the bendingmotion.

Referring to FIG. 4, a cross-sectional view of an alternative embodimentof the connecting element 12 according to this invention is shown inwhich the shanks 38 are connected to a pair of spaced outer flanges 42between which the flexible brace 40 is positioned. The central member 36is embedded within the brace 40 and includes an additional pair ofspaced inner flanges 48 between which a portion of the central member 36is held by mechanical crimping tabs 44 or similar arrangement. Each ofthe inner flanges 48 is mounted and spaced from an associated outerflange 42 by an extension 50 of the shank 38 extending into the brace40. The embodiment of FIG. 4 allows the shanks 38 and flanges 42 whenbent to apply both compressive and tensile forces to the brace 40 aswell as tension to the central member 36. The central member 36according to various embodiments of this invention may be made from aDacron™ tape woven cord, a braided stainless steel or titanium cable,carbon fiber, Kevlar™ or a variety of other materials providingsufficient strength with low elongation characteristics. Other optionsfor the central member 36 include a synthetic or natural rubber wovencord which is stretched between the flanges 42 prior to attachment andmounting therein. The cord would be attached to the flanges 42 and thenstretched into a mold cavity where the polymer or elastomer bracematerial would be injected into the mold so that the central cord wouldbe in constant tension, but below its elastic limit.

A further alternative embodiment of the connecting element 12 accordingto this invention is shown in FIG. 5 in which the central member 36 issurrounded by the brace 40 and oppositely extending shanks 38 mount theconnecting element 12 to the respective vertebral anchors 14. Thecentral member 36 of FIG. 5 may be en elastic mesh or woven member andthe polymer material of the brace 40 would then be injection moldedthrough the mesh and around the corded central member. The centralmember 36 could be steel, titanium, nickel titanium alloy, polymer,carbon fiber or other materials. In the various embodiments, theconnecting element 12 is a flexible member that provides a shockabsorbing effect in transmitting spinal column loads between the anchors14 to which it is engaged. Intermediate portion 46 between the flanges42 can also permit relative movement to allow motion of the spinalcolumn segment to which connecting element 12 is engaged. In oneembodiment, intermediate portion 46 provides connecting element 12 witha variable stiffness profile between anchors 14.

For installation requiring more than one level of treatment, a two-levelsection system 10 such as the embodiment shown in FIG. 6 could beutilized. The construct of FIG. 6 provides two flexible connectingelements 12 with a predetermined space between them. The length of theshank 38 positioned between the connecting elements 12 of the embodimentof FIG. 6 could be fixed or a variable distance to accommodate differentvertebral anatomies.

A further alternative embodiment of a spinal stabilization stiffeningsystem 10 according to this invention is shown in perspective view inFIG. 7 and cross-sectional view in FIG. 8. The vertebral anchor 14 ofthe embodiment of FIGS. 7 and 8 is shown as a pedicle screw having athreaded shaft 20 and a head 24. The vertebral anchor 14 in oneembodiment is a polyaxial pedicle screw similar to a device marketed byZimmer Spine, Inc. as the ST360™. The vertebral anchor 14 includes anupwardly extending toggle 62. The connecting element 12 is mounted tothe toggles 62 of the vertebral anchors 14. The connecting element 12 inone embodiment includes a central member 36 of an elastomeric orpolymeric material providing flexibility to the construct. The centralmember 36 is molded about a pair of rims 64 providing an aperture 66 sothat the connecting element 12 may be mounted downwardly onto thetoggles 62 and secured in place by a washer 68 and threaded nut 70combination as shown in FIGS. 7 and 8.

Referring to FIGS. 9-11, a further alternative embodiment of theconnecting element 12 according to this invention is shown. In thisembodiment, flanges 38 are provided with outwardly projecting annularhubs 52 and a securing element in the form of a setscrew 54 is seatedwithin a threaded aperture 56 on the hub 52 to secure the flange 42 andhub 52 arrangement to the shank 38 of the connecting element 12 andagainst the brace 40. The connecting element can be assembled pre- orintra-operatively. Once assembled, as shown in FIG. 10A, the connectingelement 12 is positioned in the top loading vertebral anchors 14 andsecured thereto by the vertebral anchor set screws 30 as shown in FIG.10B resulting in the arrangement and installation of FIG. 9. In thisembodiment, the intermediate portion 46 is of a different size thanintermediate portion 46 of FIG. 1 and occupies more of the distancebetween the securing members 30. A cross-sectional view of theconnecting element 12 and associated vertebral anchors 14 of FIGS. 9-10Bis shown in FIG. 11.

In a still further embodiment shown in FIG. 12, a multi-level construct10 is provided with a generally H-shaped hub 58 having a duct 60 therethrough for the central member 36 to extend from one connecting element12 to an adjacent connecting element 12.

The flanges 42 can be adapted to engage the flexible brace 40 therebetween. In one embodiment, flanges 42 include holes to receiveattachment means such as fasteners, sutures, threads, wires, or otherdevices to engage central member 36 to the respective flange 42. Inanother embodiment, the brace 40 is injection molded between flanges 42.The injected material can flow into holes in the central member 36 toform around central member 36 and at least partially around the flanges42 to provide engagement therewith. In still another embodiment,intermediate portion 46 is molded over flanges 42. In any embodiment,the connecting element 12 can be provided with or without the centralmember 36 and with or without the flanges 42.

In one procedure, anchor 14 is engageable to a first vertebra 16, asecond anchor 14 is engageable to a second vertebra 16 and a thirdanchor 14 is engageable to a third vertebra 16. When so engaged,intermediate portion 46 allows the vertebrae to which the anchors 14 areengaged to move or flex relative to one another, but limits extensionand flexion motion to provide a stabilizing effect.

Connecting element 12 can be guided into position between the anchors 14using an installation instrument as discussed above. The installationinstrument may include extenders extending from any one, two or three ofthe anchors 14. Other techniques contemplate insertion with an opensurgical technique, or guiding of the connecting element 12 distallyalong extenders extending proximally from one or more of the anchors 14.Connecting element 12 can be employed in fusion procedures or inprocedures employing dynamic stabilization without fusion. In fusionprocedures, fusion promoting material and/or one or more fusion devices,implants, or bone graft are placed in the disc space between adjacentvertebrae. In such procedures, a single level connecting element 12 maybe coupled between the vertebrae if dynamic stabilization is desired. Ifrigid stabilization is desired, an elongated connecting element 12 canbe provided and engaged between the vertebrae to be fused, and one ormore adjacent vertebral levels can be dynamically stabilized with theintermediate portion 46 engaged between these or more other vertebrallevels.

While in the illustrated embodiments the connecting elements 12 areshown as being adapted to extend along one or two vertebral levels, theconnecting elements 12 can also extend along three or more vertebrallevels. For example, one of the shanks 38 can include a length thatextends along multiple vertebral levels to be fused to provide rigidstabilization of these levels, while the other of the shanks 38 includesa length adapted to extend along at least one vertebral level to providea flexible intermediate portion 46 between vertebrae 16 for dynamicstabilization.

The flexible brace 40 discussed herein can be made from any suitablematerial allowing at some motion of the vertebral level along which theintermediate portion 46 is engaged. For example, the brace portions 40can be made from elastomers, polycarbonateurethane,polyetheretherketone, or other polymer material. The brace 40 can bemade from resorbable material. In still another form, the brace 40include springs, which can be made from metal or other suitablematerial.

It is further contemplated that the connecting element 12 can beprovided with a varying stiffness profiles to vary the stiffnessproperties of the system 10 and control movement of the one or moredynamically stabilized vertebral levels. Such varying stiffness profilescan be provided across the cross-section of a single flexibleintermediate portion 46 of a particular connecting element 12, orprovided between different intermediate portions 46 of a singlemulti-level connecting element 12, or provided between a number ofconnecting elements 12 in a kit where the connecting elements 12includes one or more portions with a stiffness profile that variesrelative to one or more of the portions of the other connecting elements12.

In one embodiment, the hardness characteristics of the materialcomprising the flexible brace 40 is varied. For example, the durometerof an elastomer material comprising the one or more flexible braceportions 40 may vary to allow selection and implantation of a connectingelement 12 providing the desired motion characteristics for thevertebral level.

In another embodiment, the connecting element 12 is provided with acentral member 36 that couples the first and second flanges 42 to oneanother through the flexible brace portion 40. The diameter of thecentral member 36 can be varied so that the connecting elements 12 withflexible brace portions 40 extending about a central member 36 with agreater diameter are stiffer than connecting elements 12 with a flexiblebrace portion 40 extending about a central member 36 of lesser diameter.

In a further embodiment, the central member 36 can be pre-tensioned sothat the flanges 42 are compressed against the flexible brace portion 40when engaged thereto. The amount of pre-tension can range from zero tothe tensile break strength of the central member 36. The greaterpre-tension loading of the central member 36 results in stiffer flexiblebrace portion 40 behavior since the preloading compresses the flexiblebrace portion 40 between the flanges 42.

From the above disclosure of the general principles of the invention andthe preceding detailed description of at least one embodiment, thoseskilled in the art will readily comprehend the various modifications towhich this invention is susceptible. Therefore, we desire to be limitedonly by the scope of the following claims and equivalents thereof.

1. A spinal stabilization system comprising: a first and a secondvertebral anchor each adapted to be coupled to one of a first and asecond vertebrae, each vertebral anchor including a U-shaped channeldefined between first and second receiving arms; a flexible connectingelement coupled to each of the first and second vertebral anchors andextending between the vertebral anchors, the connecting elementincluding a central member; a brace positioned about the central member;a first and a second flange on the connecting element, the flanges beingspaced on opposite sides of the brace; a first and a second shankextending in opposite directions from the first and second flanges,respectively, the first shank being coupled in the U-shaped channel ofthe first vertebral anchor with a first securing member threadedlyengaging threads of both receiving arms of the first vertebral anchorand the second shank being coupled in the U-shaped channel of the secondvertebral anchor with a second securing member threadedly engagingthreads of both receiving arms of the second vertebral anchor; a thirdsecuring element engaging the central member to secure the centralmember to the first shank.
 2. The spinal stabilization system of claim1, further comprising a fourth securing element engaging the centralmember to secure the central member to the second shank.
 3. The spinalstabilization system of claim 1, wherein the first and second flangesare integrally formed with the first and second shanks, respectively. 4.The spinal stabilization system of claim 1, wherein the brace is apolymeric, elastomeric or non-metallic material.
 5. The spinalstabilization system of claim 1, wherein the brace is positionedconcentrically around the central portion.
 6. The spinal stabilizationsystem of claim 1, wherein a third vertebral anchor is positionedintermediate the first and second vertebral anchors and adapted to becoupled to a third vertebrae; and a hub coupled to the third vertebralanchor and the connecting element.
 7. The spinal stabilization system ofclaim 6 wherein the hub further comprises: a third and a fourth flangepositioned on opposite sides of the third vertebral anchor; and a ductformed in the hub, the central member of the connecting elementextending through the duct.
 8. The spinal stabilization system of claim6, wherein the brace is positioned on a first side of the hub and asecond brace positioned about the central member is positioned on asecond side of the hub.